TAMR is expected to be one of the future generation of magnetic recording technologies that will enable recording at ˜1-10 Tb/in2 data densities. TAMR involves raising the temperature of a small region of a magnetic medium to near its Curie temperature where both of its coercivity and anisotropy are significantly reduced and magnetic writing becomes easier to achieve even with weak write fields characteristic of small write heads in high recording density schemes. In TAMR, optical power from a light source is converted into localized heating in a recording medium during a write process to temporarily reduce the field needed to switch the magnetizations of the medium grains. Thus, with a sharp temperature gradient of TAMR acting alone or in combination with a high magnetic field gradient, data storage density can be further improved with respect to current state of the art recording technology.
In addition to the components of conventional write heads, a TAMR head also typically includes an optical wave guide (WG) and a plasmon antenna (PA) or plasmon generator (PG). The waveguide serves as an intermediate path to guide light from a light source to the PA or PG where the light optical mode couples to the local plasmon mode of the PA or to the propagating plasmon mode of the PG. After the optical energy is transformed to plasmon energy, either with local plasmon excitation in the PA or with energy transmission along the PG, it is concentrated at the medium location where heating is desired. Preferably, the heating spot is aligned with the magnetic field from the write head to realize optimum TAMR performance.
A thermally assisted magnetic head structure disclosed in U.S. Patent Application Publication 2010/0103553 employs an edge plasmon mode that is coupled to a waveguide as represented in FIG. 1a. Conventional components of a magnetic recording structure are shown as a main pole 1, return pole 3, and write coils 5 formed along an air bearing surface (ABS) 8-8. The wave guide 4 guides the input optical light wave 6 toward the ABS 8-8 in the center cross-sectional view. As shown in the prospective view, plasmon generator 2 has a triangular shape and extends a certain distance from the ABS before meeting WG 4. Optical wave 6 couples to the edge plasmon (EP) mode 7 that is excited and propagates along the sharp edge 9 of plasmon generator 2 adjacent to the WG 4. Plasmon mode 7 further delivers the optical power toward the ABS and locally heats a medium (not shown) placed underneath the plasmon generator 2. A plasmon generator is typically made of noble metals such as Ag and Au that are known to be excellent generators of optically driven surface plasmon modes. The local confinement of the edge plasmon mode 7 is determined by the angle and radius of the triangle corner. In FIG. 1b, the magnetic field profile and heating profile are shown with slopes 10 and 13, respectively, that have a slight overlap along the dashed vertical line 14-14.
In TAMR recording, it is necessary to deliver a maximum amount of light intensity to the plasmon generator from a light source that may be a laser diode (LD) which is directly attached to a waveguide, or a free space light beam focused at the waveguide inlet by means of a lens. Because the waveguide has a cross-track dimension of less than a micron, alignment accuracy is critical and is preferably done in an active fashion. Alignment optimization is important since it reduces the source power required to deliver a certain plasmon wave energy at the ABS and guarantees the correct mode excitation in the waveguide. For a laser diode, it is beneficial to use a low power to improve the lifetime and reduce localized heating that could cause degradation. A low power regime also means a smaller LD that can conveniently fit into the limited space at the back end of the slider.
Alignment is typically performed during the head fabrication process in a non-write situation when the main waveguide is not used to deliver light to a near field device. One must keep in mind though that alignment components built into a TAMR head will be present during a subsequent write process and should not interfere with the main transmission mode in the central waveguide. Alignment preferably occurs before a LD is physically attached to a waveguide and is considered a one time set up procedure. However, for a focused beam that is transmitted by means of a lens and mirror, for example, it is conceivable that the alignment could be repeated more than once including during periods after TAMR head fabrication is completed.
In U.S. Patent Application 2009/0052077, a waveguide is formed on either side of a central waveguide for alignment purposes. The side waveguides extend from the ABS to the back end of the slider and are parallel to the central waveguide along its entire length. Side waveguides have a larger cross-sectional area at the ABS than the central waveguide and transmit a substantially higher light intensity than the central waveguide.
U.S. Patent Application 2009/0165285 describes a method of measuring the light intensity within the core of a waveguide by employing a light shield to block the path of light emitted through the waveguide cladding.
U.S. Patent Application 2008/0019648 discloses a method for manipulating light with tunable ferroelectric photonic devices which could be used to measure light intensity.
In U.S. Pat. No. 7,706,654, a dual waveguide configuration is disclosed where a first waveguide made of Ta2O5 or the like delivers light onto a focal region at an interface with a second waveguide comprised of metal layers and a tapered opening adjacent to the focal region.